The invention relates to methods for nondestructive evaluation of materials and more particularly to tests for photoreceptors.
A key component in many copy machines and laser printers is a photoreceptor onto which a powdered xe2x80x9cinkxe2x80x9d or toner is deposited in a precise pattern. The pattern is generated by light exposure, and corresponds to the images or letters to be copied or printed. Paper is placed into contact with the photoreceptor, toner is transferred to the paper, and the xe2x80x9cinkedxe2x80x9d paper is subjected to heat and pressure to fuse the toner onto the paper. The photoreceptor is then wiped clean of any remaining toner, and the process is repeated.
Among many possible configurations, an important commercial photoreceptor design is a multilayer structure wherein a metal drum is coated with an insulating polymer base layer, and an organic photoconductive (OPC) overcoat consisting of a charge generating layer and a charge transport layer. In the copying or laser printing process, deposition of the toner on the photoconductive drum is effected by creating a negatively or positively charged pattern on the drum to which oppositely charged particles of toner powder cling. This initial charged pattern is created by imparting a charge of the desired polarity to the entire drum. After electrostatic charging, the cylinder is carefully exposed to a pattern of light which causes charge carriers to be freed within the charge generating layer of the drum. These charge carriers, under the influence of the existing electric field, migrate through the charge transport layer, neutralize the deposited charge, thereby electrically discharging those portions. The remaining charged portions of the surface correspond to the pattern to which the toner is electrostatically attracted.
To produce clear and consistent images, the photoconductive drum must be able to consistently accept a sufficient level of charge, hold the charge for a sustained period of time in the dark, and discharge rapidly under controlled light exposure over repeated cycles. Devices presently exist which enable tests of certain charge build-up, charge retention, and discharge characteristics to be performed. However, these devices are not suitable for measuring other quality characteristics during the manufacture of new drums, or in the recycling of used drums. These characteristics include the cleanliness of the underlying metal drum surface, the uniformity of thickness of the polymer base and charge generation layers, the presence of layer defects, and various measures of electrophotographic performance across the entire drum surface.
While generalized techniques for nondestructive detection and characterization of flaws in materials, such as those described in U.S. Pat. No. 4,443,764 to Suh et al. are known, there is no automated test apparatus or series of tests that is capable of evaluating the full range of photoconductive drum characteristics that bear on print quality. Such tests and an easily operated apparatus for performing them would be important to quality control during manufacture and refurbishment of products having photoconductive drums.
Other materials such as paper and other media may be called upon to hold a charge and a system is needed for their testing as well with out expensive and time consuming procedures.
The present invention provides a fully automated test system capable of performing a comprehensive series of tests to evaluate the numerous characteristics of the various layers of a photoreceptor, such as a photoconductive drum, which affect its print performance. The tests include complete electrophotographic characteristic evaluation, defect mapping, layer thickness measurement and uncoated drum substrate cleanliness testing.
The user can select and test for the cleanliness of the uncoated drum surface; the thickness of the drum""s base coating polymer (with or without a charge generating layer thereon) and of the charge generating layer itself; the presence of defects in the base, charge generating, transport layers; as well as the charge build-up, charge retention and discharge characteristics of the completed drums.
A further feature of the invention is a test system including a control unit responsive to user defined parameters for controlling preprogrammed tests; a test station including a drum retainer for holding and rotating the drum, a charging system having a high voltage power supply and a polarity selector, a light source for causing localized electrostatic discharge, a broad spectrum light source for causing global electrostatic discharge, and a low voltage and a high voltage non-contact electrostatic surface potential probe; a light-proof cabinet for enclosing the test station; an adjustable fixture for the charging system, high voltage and low voltage probes and electronics therefor for drum scanning, a mounting ring to provide convenient positioning and adjustment of the probes and light source; and motors to produce drum rotation and ring movement; and an output device for presenting test results to a system operator.
The drum substrate surface cleanliness test includes completely scanning the drum by the low voltage electrostatic sensor to measure contact potential values or the surface potential on the surface of the drum to detect surface contamination.
The layer thickness test can be run on a drum having a base polymer layer with or without the charge generating layer thereon. By appropriate use of charge polarity and discharge light, the thickness of each layer is determined from the differences in charge mobility and photosensitivity of the two layers.
The defect test is typically run on a fully layered drum. The drum is charged during a scan and the resulting potential on the drum surface is immediately measured. In this test, different combinations of test conditions including the light exposure level and charge polarity provide indications of different types of defects.
The electrophotographic measurement test is conducted on a fully layered drum. The system continuously monitors the potential on the drum surface at a predefined set of locations while sequentially performing each of the following operations for a predefined time duration: imparting an electrostatic charge to the drum; allowing the charge to decay in the dark; and finally illuminating at least a portion of the drum surface with light of a preselected wavelength and intensity to cause electrostatic discharge. Parameters which characterize the electrophotographic performance of the drum are obtained and/or computed from the acquired data. The entire test sequence is repeated many times to perform a cyclic fatigue test of the drum.
A further feature of the invention is a test system including a charging device, a voltage measurement probe, and a light source that are conveyed linearly along a photoconductive drum for performing voltage acceptance and discharge testing.
Yet another feature of the invention is a life-cycle test for comparing voltages imparted to a drum and residual voltages after discharge by a light source to life-cycle reference voltages for a similar drum.
In other features of the invention, embodiments are provided for the testing of substrates in the nature of paper and other media, for testing both voltage retention and current acceptance, and for the dielectric relaxation process. Corona electrodes or capacitive arrays, rollers and wires are utilized for applying electrical test energy.